Fuel cell stack with internal gas connections

ABSTRACT

The invention relates to a fuel cell stack with a plurality of plate shaped fuel cells. Each fuel cell provides an electrolyte layer, an adjacent electrode and cathode. The fuel cells are connected mechanically and electrically by bipolar plates. Tightness is ensured in an improved manner. The fuel cells provide a cathode, an anode and an electrolyte layer arranged between the cathode and anode. Each cathode is adjacent to a cathode chamber and each anode is adjacent to an anode chamber. The bipolar plates comprise first openings which are used to provide channels to allow operating material to be transported to the fuel cells or to enable the removal of depleted operating material. The bipolar plates have additional openings, in which the fuel cells are arranged. When the fuel cell is disposed in the opening, it is only substantially and openly accessible from one side. This openly accessible side or surface is essentially formed by a fuel cell electrode. Sealing material is arranged layer-by-layer above the other opening. At least one portion of the edge of the fuel cell disposed in the opening is covered by the sealing material.

DISCLOSURE

The invention relates to a fuel cell stack. Each fuel cell provides anelectrolyte layer, an adjacently arranged anode and cathode. The fuelcells are connected mechanically and electrically by bipolar plates.Together with an adjacent electrode, each bipolar plate encloses anelectrode chamber. If the electrode is an anode, the chamber is referredto as an anode chamber. Similarly, the chamber adjacent to the cathodeis referred to as the cathode chamber. Each electrode chamber providesopenings, which are used for the supply or the removal of an operatingmaterial.

During operation of the fuel cell, an operating material is fed intoeach electrode chamber. The fuel is either a fuel such as hydrogen or anoxidation agent, such as oxygen or air. Ions of an operating material,that is, either oxygen ions or hydrogen ions, penetrate the electrolytelayer. The hydrogen ultimately combines with the oxygen to form waterwith the release of electrical current.

BACKGROUND OF THE INVENTION

Gas-tight separation of the cathode chamber from the anode chamberrepresents a substantial problem with a fuel cell stack. If an operatingmaterial from the anode chamber were able to enter the cathode chamberdirectly (or vice versa), the consequence would be an oxygen-hydrogenreaction.

The problem of tightness is particularly serious in the case ofso-called high temperature fuel cells, because, in addition totemperature loading, thermal stresses also result from the heating andcooling of the fuel cells.

A fuel cell stack of the type described above is disclosed in thespecification DE 40 09 138 A1. A plurality of openings are incorporatedwithin the bipolar plates, which are adapted to one another in such amanner that supply channels and/or outlet channels for operatingmaterials are formed as a result.

A fuel cell stack in which at least one electrode is sufficiently thickto be self-supporting is already known. In this context, self-supportingis understood to mean that such an electrode substantially retains itsform if the horizontally orientated electrode is held by one corner orone edge and lifted. The portion of the electrode which is not held doesnot bend as a result of gravity.

A self-supporting electrode of this type may, for example, be 1.5 mmthick. In order to save materials and ensure the functional efficiencyof the electrode during operation, the electrode should be as thin aspossible. In view of this requirement, an electrode for high temperaturefuel cells according to the current state of technology must be at least0.5 mm thick.

The provision of a self-supporting electrode allows the application of avery thin electrolyte layer to the electrode. A thin electrolyte layeris desirable, because this can then be penetrated rapidly andextensively by the ions of an operating material. The performance of afuel cell can be improved in direct proportion to the number of ionswhich can pass through the electrolyte layer.

The problem of tightness in a fuel cell stack is particularly difficultwith fuel cells of this type which provide a considerable overallthickness in view of the above-mentioned design.

With reference to fuel cells with self-supporting electrodes, it isalready known that channels can be provided from which the operatingmaterial passes into the individual electrode chambers of a fuel cellstack. By contrast with the prior art, which is known from thespecification DE 40 09 138 A1, these channels are formed from separatestructural elements. This is understood to mean that openings for theformation of channels according to specification DE 40 09 138 A1 are notprovided in the case of fuel cells with self-supporting electrodes.

The provision of separate structural elements increases the problem ofgas tightness because, in this case, an additional component must besealed, which, under some circumstances, consists of a further material,different from that which has already been used. Additional thermalstresses may result from this.

The object of the invention is to create a fuel cell stack of the typenamed in the introduction, in which tightness is ensured in an improvedmanner.

The object of the invention is achieved with a fuel cell stack with thefeatures of claim 1. Advantageous embodiments are described in thedependent claims.

SUMMARY OF THE EMBODIMENTS OF THE INVENTION

The fuel cell stack according to the claims comprises a plurality ofplate-shaped fuel cells, which are mechanically and electricallyconnected by bipolar plates. Each fuel cell comprises a cathode, ananode and an electrolyte layer disposed between the cathode and theanode. Each cathode is adjacent to a cathode chamber, and each anode isadjacent to an anode chamber. The bipolar plates provide first openingsthereby forming channels for the supply of operating materials to thefuel cells or for the removal of depleted operating materials. Firstopenings of this kind are already known from the specification DE4009138 A1.

However, the bipolar plates provide additional openings, in which thefuel cells are disposed. The dimensions of these additional and/or otheropenings are substantially adapted to the dimensions of the fuel cell.When the fuel cell is disposed in the openings, it is only substantiallyand openly accessible from one side. This openly accessible side and/orsurface is essentially formed by an electrode of the fuel cell. Sealingmaterial is disposed layer-by-layer above the other openings, so that atleast a portion of the edge of the fuel cell disposed in the openings iscovered by the sealing material. It suffices if less than 1 mm of thissurface of the fuel cell is covered at the edge. Moreover, this sealingmaterial is disposed layer-by-layer on the adjacent surface of a bipolarplate in such a manner that the desired extent of sealing is provided.With regard to sealing, attention should be paid to the fact that thevarious openings, which form the supply and/or outlet channels, areconnected in an appropriately gas-tight manner. Furthermore, particularattention should be paid to the fact that a cathode chamber of a fuelcell is separated in a gas-tight manner from the anode chamber of thisfuel cell. By providing an opening and extending the sealing materialover the edges of the fuel cell, a particularly reliable seal isprovided. Thermal stresses resulting from different materials are nolonger evident in practice, because the sealing material is disposedpredominantly between two bipolar plates, which naturally provideidentical coefficients of expansion. The different coefficients ofexpansion of the fuel cell no longer play any part in practice because,in this context, sealing is achieved by a slight overlap. The sealingmaterial is not in contact with the fuel cell over a large area, which,with different coefficients of thermal expansion, would be particularlyproblematic.

Moreover, the above structure is simple to manufacture, so that thedevice according to the claims also provides no problems from thisaspect.

The above advantages with reference to tightness can be achieved inparticular with fuel cell stacks using fuel cells with at least oneself-supporting electrode. A self-supporting electrode of this kindgenerally provides a thickness of at least 0.5 mm. The electrodepreferably should not exceed a thickness of 1.5 mm, amongst otherreasons, in order to prevent excessive expenditure on materials. Atypical thickness may, for example, be 1 mm.

In practice, it has proved successful to provide the anode as theself-supporting electrode.

Moreover, the advantages with reference to tightness are especiallyachieved in the case of fuel cells provided for use at hightemperatures. The invention is therefore advantageously used forhigh-temperature fuel cells. High-temperature fuel cells are operated attemperatures above 600° C. In general., a temperature of 1000° C. is notexceeded in high-temperature fuel cells. Typical temperatures arecurrently in the region of 800° C.

In one embodiment of the invention, glass solder is used as the sealingmaterial. Glass solder tolerates the operating conditions, which occurin a high-temperature fuel cell. It also tolerates the thermallydetermined changes which are unavoidable in a high-temperature fuelcell.

The fuel cell used in the opening of a bipolar plate advantageouslyterminates flush with the surface of the bipolar plate onto which thesealing material is applied. A tolerance of approximately ±0.2 mm hasproved acceptable in practice. If the above-named surfaces of the fuelcell and/or the bipolar plate are disposed at the same height, thedesired tightness between the cathode chamber and the electrode chamberis more readily achieved in a particularly reliable manner. Slightdeviations within the context of the above-named tolerances only impairthe desired effect to an insignificant extent.

In one further embodiment of the invention, a metallic mesh is providedbetween the base of the opening and the fuel cell disposed in theopening. The mesh establishes the electrical contact between theelectrodes of the fuel cell adjacent to the mesh and the base of thebipolar plate. Furthermore, manufacturing tolerances can be compensatedthrough the mesh.

A rectangular or square fuel cell provides two main surfaces and fourlateral edges. In the above-named basic form, one lateral edge passes atright angles into the next lateral edge. By preference, the two opposingedges of each fuel cell are framed by bipolar plates. The bipolar platesthen form an indentation, into which the relevant edge of the fuel cellis pressed. On one side of a principal surface of the fuel cell, theedge is separated from the opposing bipolar plate by the sealingmaterial. In this manner, the fuel cell is incorporated into the fuelcell stack in a mechanically stable manner.

To the accomplishment of the foregoing and related ends the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims, the following description andthe annexed drawings setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however, of but afew of the various ways in which the principles of the invention may beemployed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical section of a fuel cell stack in accordance with thepresent invention; and

FIG. 2 is a partial enlarged section of one cell, normal to the plane ofFIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be explained with reference to FIGS. 1 and 2.

FIG. 1 shows a fuel cell stack, which comprises two fuel cells. Eachfuel cell provides a self-supporting anode electrode 1. An electrolytelayer 2 is applied to the self-supporting electrode 1 and, above that, acathode 3 is applied. Bipolar plates 4 connect the two fuel cellselectrically and mechanically to each other. The bipolar plates areprovided with openings 5 and 6 in such a manner that, as a result,channels are formed for the supply and removal of operating materials.The four openings 5 shown in FIG. 1 originate either from a front or arear cutting plane. Furthermore, the bipolar plates are provided withgrooves at the sides, which are adjacent to the electrodes of the fuelcells. The anode chambers 7 and cathode chambers 8 are formed by thesegrooves. Each fuel cell is introduced into a further opening of anassociated bipolar plate. The side of the fuel cell facing the openingterminates substantially flush with the neighbouring surface of thebipolar plate. Sealing material 9 and 10 is disposed layer-by-layerand/or strip-by-strip above the fuel cell and the bipolar plate. Thesealing material indicated by reference number 10 is disposed above thebordering edge between the bipolar plate 4 and the fuel cell which isintroduced into the opening of the bipolar plate. As illustrated thesealing material projects beyond the edge of the openings to ensure thatit is covered by the sealing material. The sealing material 10 thereforeextends from the corresponding principal surface of the fuel cell up tothe neighbouring surface of the bipolar plate. This means that, inpractice, the thermal changes along one principal surface of the fuelcell exert no further damaging influence on the portion of the sealwhich is indicated by reference number 10.

FIG. 2 provides an enlarged detail of the fuel cell stack shown insection in FIG. 1. Furthermore, the section according to FIG. 2 isrotated through 90° by comparison with the section from FIG. 1.

According to FIG. 2, a fuel cell is disposed in an opening 11 of abipolar plate 4. Between the base of the opening 11 and the fuel cell, ametallic mesh 12 is shown. The bipolar plate 4 adjacent to the otherside of the fuel cell is provided with a contact layer 13. An electricalcontact between the bipolar plate and the adjacent cathode 3 isestablished through the contact layer 13.

The framing of one edge of one fuel cell is illustrated in FIG. 2. Theedge of the fuel cell formed by the anode 1 projects into an indentationwhich originated from the form of the bipolar plates. The indentation isa consequence of the opening 11. The edge of the fuel cell projects intothis indentation and is framed in the sense of the present invention.

The diagrams relate to an embodiment, in which the flow passes throughthe cathode chamber and the anode chamber in the same direction. With anappropriate embodiment, it is, of course, also possible for the flow topass through the cathode chamber transversely to the direction of flowin the anode chamber.

1. A fuel cell stack with a plurality of plate-shaped fuel cells, whichare connected mechanically and electrically by bipolar plates (4); eachfuel cell stack comprises an anode (1), a cathode (3) and an electrolytelayer (2) disposed between the cathode and the anode; each cathode isadjacent to a cathode chamber (8) and each anode is adjacent to an anodechamber (7); the bipolar plates provide including openings arranged toform channels (5, 6) for the supply of operating materials to the fuelcells or for the removal of depleted operating materials from the fuelcells; each bipolar plate including at least one further opening (11) inwhich a fuel cell is disposed; the dimensions of the at least onefurther opening are substantially adapted to the dimensions of the fuelcell; and sealing means (10) arranged layer-by-layer or strip-by-stripcovering at least one bordering edge between the fuel cell and theadjacent surface of the bipolar plate in which the fuel cell isdisposed, said sealing means projecting beyond the edge to ensure thatit is covered.
 2. A fuel cell stack according to claim 1, wherein thecathode and/or the anode provides a thickness such that the cathode orthe anode is self-supporting.
 3. A fuel cell stack according to claim 1,wherein the anode is at least 0.5 mm thick.
 4. A fuel cell stackaccording to claim 1, wherein two bordering edges between the fuel celland the adjacent surface of the bipolar plate in which the fuel cell isdisposed are covered by the sealing means.
 5. A fuel cell stackaccording to claim 1, wherein the sealing means comprises a strip atleast 0.1 mm wide of sealing material, arranged layer-by-layer orstrip-by-strip and covering the bordering edge between a fuel cell and abipolar plate, and is arranged above the fuel cell.
 6. A fuel cell stackaccording to claim 5, wherein said sealing material is selected toenable use at temperatures above 600° C., and preferably above 800° C.7. A fuel cell stack according to claim 6, wherein a glass solder isused as a sealing material.
 8. A fuel cell stack according to claim 1,wherein the cathode chambers and anode chambers are formed by thebipolar plates and the cathodes and/or anodes.
 9. A fuel cell stackaccording to claim 1, wherein a principal surface of a fuel cellterminates flush with the surface of the bipolar plate which is adjacentto the further opening.
 10. A fuel cell stack according to claim 1,wherein a metallic mesh is arranged layer-by-layer in at least one ofthe further openings between the base of the opening and the fuel celldisposed in the opening.
 11. A fuel cell stack according to claim 1,wherein at least two edges of each fuel cell are framed by said bipolarplates.